/*
 * n(n+1)/2 + 1 = [n(n+1) + 2]/2 = (n*n + n + 2)/2
 */

 #include <stdio.h>


 int main ()
 {
 	long long int cuts;

 	while (scanf("%lld", &cuts), cuts >= 0) {
 		cuts = (cuts * cuts + cuts + 2) / 2;
 		printf("%lld\n", cuts);
 	}

 	return 0;
 }